(1) Field of the Invention
The invention relates to the general field of field emission displays with particular reference to problems of image smearing.
(2) Description of the Prior Art
Cold cathode electron emission devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small conical emitters, each of which is connected to a source of negative voltage via a cathode conductor line or column. Another set of conductive lines (called gate lines) is located a short distance above the cathode lines at an angle (usually 90.degree.) to them, intersecting with them at the locations of the conical emitters or microtips, and connected to a source of relatively positive voltage.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike a layer of phosphor that is located some distance above the gate lines. Thus, one or more microtips serves as a sub-pixel for the total display. The number of sub-pixels that will be combined to constitute a single pixel depends on the resolution of the display and on the operating current that is to be used. In general, even though the local electric field in the immediate vicinity of a microtip is in excess of 1 million volts/cm., the externally applied voltage is under a 100 volts.
A number of factors affect the sharpness of the images that are formed in displays of this type, for example the degree to which the electron beam diverges after it has passed through the gate electrode. A problem, known to be associated with this type of display, is that of `smearing` where an otherwise sharp image appears to be surrounded by a diffuse halo of light. The origins of this defect are not entirely clear but our own investigations suggest that it is due to spurious reflections from the surface of the gate electrode layer.
We will amplify this by reference to FIG. 1. Seen there is a schematic cross-section of a cold cathode display of the type that we have been discussing above. Cathode electrode 11 (normally in the form of extended columns) lies on lower dielectric substrate 10. Immediately above cathode 11 is dielectric layer 12 which serves to support gate electrode 13 (normally in the form of rows running at right angles to the cathode columns) as well as to electrically insulate it relative to 11. Holes, such as 18, have been formed in the gate electrode and these holes extend down to the surface of cathode layer 11. In each such hole a conical microtip, made of material such as molybdenum or silicon, is seated. Positioned some distance above the microtips by means of insulating spacers (not shown) is upper dielectric substrate 16 on whose downward facing surface layer 15 of transparent conducting material, indium tin oxide (ITO), has been deposited. The ITO in turn is covered with layer 14 of a suitable phosphor which will emit light in some desired wavelength range when it is struck by electrons coming from the microtips.
Continuing our reference to FIG. 1, we show there a phosphor particle 21 that, having been subjected to bombardment by electrons coming from microtip 19, emits phosphorescent light rays 22 in all directions, both outwardly (and hence seen as part of the display) and inwardly where the majority of them are lost and not seen by an external viewer. However, a small fraction of rays 22, represented in the figure as ray 23, arrive at the surface of gate electrode layer 13. The latter is typically made of niobium or molybdenum and provides a good reflecting surface. The resulting reflected ray (shown as 24 in the figure) is then returned to the upper substrate, passing through phosphor layer 14 on its way. As it passes through the phosphor layer, ray 24 may get diverted by refraction. The net result is the emergence of rays 25 which give an outside viewer the impression that they originated from microtip 20 instead of from microtip 19. This we believe to be the origin of the smearing phenomenon discussed above.
In the prior art, as far as we are aware, the only way in which the smearing problem has been dealt with has been to increase the thickness of the phosphor layer. This is illustrated in FIG. 2 which can be seen to be the same as FIG. 1 except that phosphor layer 114 is substantially thicker than corresponding phosphor layer 14 in FIG. 1. The result of this change is that reflected ray 24 is now subject to significant attenuation on its way to the surface so that the cone of emitted light 125 which is visible to an external viewer is significantly fainter than corresponding cone 25 in FIG. 1. While this approach does reduce the amount of smearing, it does so at the cost of a fainter image since the light associated with a given electron has more material to penetrate on its way to the surface.
Wei et al. (U.S. Pat. No. 5,517,031 May 1996) shows a photosensor array where the photosensors are backed up by an opaque layer to eliminate false imaging effects. Hashimoto (U.S. Pat. No. 5,478,611 December 1995) describes a type of black matrix for an LCD display while Kim (U.S. Pat. No. 5,338,240 August 1994) also describes a black matrix for an LCD display based on using two substrates.